I. Field of the Invention
The present invention relates generally to spread spectrum communications systems, and more particularly to enabling multiple transmitters to share a single code divis on multiplexed (CDM) or code division multiple access (CDMA) channel, as a shared resource in such systems.
II. Description of the Related Art
In a code division multiplexed (CDM) system, signals intended for one or more recipients are transmitted from a single site using a single frequency band, or CDM channel, through the proper assignment of channelizing codes to create code channels. Such systems include, for example, paging systems, message or information broadcast systems, and positioning or position determination systems in which information is transferred to various targeted recipients. Some CDM systems, such as spread spectrum code division multiple access (CDMA) communication systems obtain code channels by assigning orthogonal channelizing codes, such as Walsh codes, or spreading codes with low correlation to each system user.
A variety of multiple access communication systems and techniques have been developed for transferring information among a large number of system users. However, spread spectrum modulation techniques, such as used in (CDMA) communication systems provide significant advantages over other modulation schemes, especially when providing service for a large number of communication system users. Such techniques are disclosed in the teachings of U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990 under the title "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", and U.S. patent application Ser. No. 08/368,570 filed under the title "Method And Apparatus For Using Full Spectrum Transmitted Power In A Spread Spectrum Communication System For Tracking Individual Recipient Phase Time And Energy," which are both assigned to the assignee of the present invention, and incorporated herein by reference.
The above-mentioned patents disclose multiple access communication systems in which a large number of generally mobile or remote system users each employ at least one transceiver to communicate with other system users or users of other connected systems, such as a public telephone switching network. The transceivers communicate through gateways and satellites, or terrestrial base stations (also sometimes referred to as cell-sites or cells).
Base stations cover cells, while satellites have footprints on the surface of the Earth. In either system, capacity gains can be achieved by sectoring, or subdividing, the geographical regions being covered. Cells can be divided into "sectors" by using directional antennas at the base station. Similarly, a satellite's footprint can be geographically divided into "beams", through the use of beam forming antenna systems. These techniques for subdividing a coverage region can be thought of as creating isolation using relative antenna directionality or space division multiplexing. In addition, provided there is available bandwidth, each of these subdivisions, either sectors or beams, can be assigned multiple CDMA channels through the use of frequency division multiplexing (FDM). In satellite systems each CDMA channel is referred to as a "sub-beam", because there may be several of these per "beam".
In a typical spread-spectrum communication system, one or more preselected pseudonoise (PN) code sequences are used to modulate or "spread" user information signals over a predetermined spectral band prior to modulation onto a carrier signal for transmission as communication signals. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a sigial for transmission that has a bandwidth much greater than that of the data signal. In the base station- or gateway-to-user communication link, PN spreading codes or binary sequences are used to discriminate between signals transmitted by different base stations or over different beams, as well as between multipath signals. These codes are typically shared by all communication signals within a given cell or sub-beam.
In a typical CDMA spread-spectrum communication system, channelizing codes are used to discriminate between different users within a cell or between user signals transmitted within a satellite sub-beam on a forward link (i.e., the signal path from the base station or gateway to the user transceiver). That is, each user transceiver has its own orthogonal channel provided on the forward link by using a unique `channelizing` orthogonal code. Walsh functions are generally used to implement the channelizing codes, with a typical code length for the forward link being on the order of 64 code chips for terrestrial systems and 128 code chips for satellite systems.
In general, a CDMA satellite system makes an assignment of system resources to the many gateways. The simplest assignment scheme is to divide up resources at the resolution of whole CDMA channels, or sub-beams. The system assigns whole sub-beams of individual satellites to individual gateways for specified periods of time. However, when there are many more gateways than available sub-beams, the assignment of whole CDMA channels potentially becomes inefficient in using system resources. In such situations, it could prove useful to share a sub-beam between gateways. This increases the resolution of system resources available for assignment.
Therefore, it is desirable for multiple gateways to share one CDMA channel or sub-beam as a shared resource. However, according to conventional wisdom, the sharing of a CDMA or CDM channel by multiple transmitters results in signal interference at the receivers. It will be apparent to one skilled in the art that this discussion also applies to terrestrial (e.g., cellular) communications systems employing base stations rather than gateways, and several types of message or information broadcast systems.
What is needed, therefore, is a method for enabling multiple transmitters (e.g., gateways, base stations to share a single CDM channel without the creation of interference.